Forester Remap????
Discussion
Here goes, I have a 2001 Subaru Forester which I had to put a new CAT (non Genuine) on last November, due to it rattling. Within a month the Engine light came on, was diagnosed as the rear 02 sensor in the CAT. Light extinguished all was well for a couple of weeks until the light came back on. I thought it would be the forward sensor in the CAT, but Subaru say that's fine and it's the CAT. Their equipment saying " CAT below threshold". So I went back to the place I got the CAT changed and explained. They changed it again without hesitation, brilliant i thought. Light is back on, Subaru insist the other sensor is fine and that it's probably due to it being a non genuine CAT and the computer is registering it "below threshold". What i'm wondering is, if the CAT is fine and is doing it's job, would an engine remap extinguish the light on the dash and tell the computer that this CAT is ok and to accept these tolerances. A long one I know, but any advice greatly appreciated.
You may be able to pick up a cat second hand somewhere. But it wouldnt be the first time an aftermarket cat has been useless, when an OEM cat works.
Not sure how local etc. There's a guy Craig McD in Paisley. Seedhill Garage in Paisley.
Dont ahev a number, just googled the name. He's a regular member over on the UK Legacy forum, although I havent frequented it for a few months.
http://www.uklegacy.com/forums/index.php?s=0935fad...
Not sure how local etc. There's a guy Craig McD in Paisley. Seedhill Garage in Paisley.
Dont ahev a number, just googled the name. He's a regular member over on the UK Legacy forum, although I havent frequented it for a few months.
http://www.uklegacy.com/forums/index.php?s=0935fad...
If the CAT is not cleaning up the pollutants in the exhaust gas properly and the second sensor is producing readings that mirror the first sensor there is nothing you can achieve with a remap to alter that.
You need to understand how the sensors work. The 1st one (upstream of the CAT) is the one that adjusts the fueling to maintain a stoichiometric A/F ratio of about 14.7. It has no idea what the CAT or the second sensor are doing and can NOT trigger a fault code if either of those fail or even if they don't exist. It just monitors the oxygen content of the pre-CAT gases and adjusts the ecu output as necessary. Its output should be a constantly varying signal switching from low voltage (0.2V) to high voltage (0.8v or so) several times a second as it juggles the fueling.
The reason for this is that sensor voltage output is not linear with oxygen content of the exhaust gas. If the mixture is even just a tad weak the voltage drops right down to about 0.1 or 0.2V. If even a tad rich it shoots up to 0.7 or 0.8V. The target is a mean of about 0.45V but that is like balancing a pencil on its point. All the sensor can do is add a tiny bit of fuel to a weak mixture which will instantly generate a rich reading so it knocks that tiny bit of fuel back off again and so on ad infinitum. The amount of fuel added and subtracted is so small it makes no difference to the engine operation.
You can remap the ecu to your heart's delight but the 1st sensor will override all of that and continue to maintain a 14.7:1 A/F ratio except at full throttle when the sensor system is bypassed.
The second sensor monitors the CAT which should clean up any free oxygen as it burns the other by-products of combustion. If the CAT is working properly it should show a fairly steady reading of 0.45V or more indicating little or no free oxygen left in the exhaust gases. If the CAT is not working then the 2nd sensor readings will just mirror the 1st sensor and that's when the ecu triggers a fault code. The 2nd sensor plays no part in adjusting the fueling though. It only monitors the post-CAT oxygen level.
You clearly have a CAT that is not doing its job properly. It will make not a jot of difference to how the engine runs though as that is purely determined by the 1st sensor. If you want the engine light off then buy a decent brand of CAT. If not then just live with it. You should still pass an MOT if the 1st sensor is working properly.
You need to understand how the sensors work. The 1st one (upstream of the CAT) is the one that adjusts the fueling to maintain a stoichiometric A/F ratio of about 14.7. It has no idea what the CAT or the second sensor are doing and can NOT trigger a fault code if either of those fail or even if they don't exist. It just monitors the oxygen content of the pre-CAT gases and adjusts the ecu output as necessary. Its output should be a constantly varying signal switching from low voltage (0.2V) to high voltage (0.8v or so) several times a second as it juggles the fueling.
The reason for this is that sensor voltage output is not linear with oxygen content of the exhaust gas. If the mixture is even just a tad weak the voltage drops right down to about 0.1 or 0.2V. If even a tad rich it shoots up to 0.7 or 0.8V. The target is a mean of about 0.45V but that is like balancing a pencil on its point. All the sensor can do is add a tiny bit of fuel to a weak mixture which will instantly generate a rich reading so it knocks that tiny bit of fuel back off again and so on ad infinitum. The amount of fuel added and subtracted is so small it makes no difference to the engine operation.
You can remap the ecu to your heart's delight but the 1st sensor will override all of that and continue to maintain a 14.7:1 A/F ratio except at full throttle when the sensor system is bypassed.
The second sensor monitors the CAT which should clean up any free oxygen as it burns the other by-products of combustion. If the CAT is working properly it should show a fairly steady reading of 0.45V or more indicating little or no free oxygen left in the exhaust gases. If the CAT is not working then the 2nd sensor readings will just mirror the 1st sensor and that's when the ecu triggers a fault code. The 2nd sensor plays no part in adjusting the fueling though. It only monitors the post-CAT oxygen level.
You clearly have a CAT that is not doing its job properly. It will make not a jot of difference to how the engine runs though as that is purely determined by the 1st sensor. If you want the engine light off then buy a decent brand of CAT. If not then just live with it. You should still pass an MOT if the 1st sensor is working properly.
AlaskanMal said:
Thanks guys, alot of information to go on there. Will probably just look into the re-flash, car seems to run ok with light on, but it just bugs me seeing it on every time you look down. Thanks again.
Stick a bit of black tape over it or just fix the problem.Reflash will cost several hundred pounds and be a waste of money.
You can just fit the downstream cat in a threaded spacer, which keeps the ceramic active element out of the main exhaust stream. This means the sensor does not see many switching events (because it ends up "becalmed" in it's own little vortex of exhaust gas) Effectively, you just space the sensor about 1" up out of the exhaust.........
(BTW, the downstream sensor DOES trim the engines fuel target, because it is aiming to keep the average catalyst oxygen content switching either side of stoichiometric. This so called "bias" trim is small (usually clipped in the order of 2 to 5% maximum) and is there to account for catalyst ageing (and hence oxygen storage capacity))
(BTW, the downstream sensor DOES trim the engines fuel target, because it is aiming to keep the average catalyst oxygen content switching either side of stoichiometric. This so called "bias" trim is small (usually clipped in the order of 2 to 5% maximum) and is there to account for catalyst ageing (and hence oxygen storage capacity))
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